1. Field of the Invention
The present invention relates to semiconductor laser devices and optical modules employing them. In particular, the invention relates to low power consumption light emitting/receiving semiconductor devices suitable for optical communications and the like.
2. Description of the Related Arts
With the rapid development of information technology, not only conventional communication (telecommunication) networks but also SAN (Storage Area Network), Ethernet (LAN) and other data communication networks, which have been progressed remarkably, are growing in system capacity. Accordingly, it is becoming important more and more to allow more channel ports to be installed economically. The throughput of a high speed interface apparatus is limited by the speed of each channel port and the installed channel port density which depends on the size and power consumption of each channel port module. Therefore, reducing the size and power consumption of its main part, namely an optical component is now the key which determines the total performance of such an apparatus.
Thus, lower power consumption has become an important goal for optical transceiver modules each of which transmits and receives optical signal by its internal semiconductor laser and semiconductor photo detector.
In FIG. 1, transmitter light sources used for optical transceiver modules at 10 Gbps operation are classified by power consumption and fiber link distance. The figure indicates the existence of a tradeoff relation between the fiber link distance and power consumption of each module. This is because a different optoelectronic device is used for each distance range as shown in the figure. In short distance applications, directly modulated 1.3 and 0.85 μm wavelength band devices are typically used to transmit optical signals. In more detail, 0.85 μm wavelength band multimode vertical cavity surface emitting lasers (Non-patent Document 1: “2003 IEEE 16th LEOS Conference Digest (Lasers and Electro-Optics Society 16th Annual Meeting of the IEEE)”, Volume 2, 27-28 October pp. 511-512) are used as directly modulated light sources for very short distance transmissions up to 100 m. In short distance applications up to 10 km, 1.3 μm band single mode edge emitting lasers (Non-patent Document 2: “2003 Optical Fiber Communications Conference (OFC) Digest”, 3-28 March, PD40) are used as directly modulated light sources. The direct modulation method can realize a low power consumption module because the module's structure is relatively simple. In particular, surface emitting lasers are substantially superior in power saving since their micro cavity structure, shorter than 1 μm, can operate at a very small current of several to ten mA. They are also economically superior since their lasing structure can all be fabricated by wafer process and on-wafer testing/sorting is possible. In the case of edge emitting lasers for the 1.3 μm wavelength band, since a minimum of about 60 mA must continue to be injected for a state of the art edge emitting laser to operate at 10 Gbps, consuming roughly twice as much power. Therefore, research is earnestly being carried out in order to apply 1.3 μm wavelength band surface emitting lasers in short distance applications up to 10 km. However, it is technically still difficult to realize a single transverse mode structure chip capable of outputting several mW level optical power although this must be cleared for practical use. This is because the volume of the light emitting layer is too small. As shown in the above-cited Non-patent Document 2, a typical 1.3 μm wavelength band surface emitting laser sharply reduces its optical output power to about several hundred pW at high temperature if the emitting area size is designed to be 5 μm or smaller to secure single mode operation. Although it is possible to attain about 1 mW output power by enlarging the emitting area size to the order of 10 μm, this causes multimode operation.
In middle and long distance applications beyond 40 km, 1.55 μm wavelength band lasers are used and externally modulated by optical modulators which operate in the 1.55 μm band. This reduces the fiber optic transmission loss but results in increased power consumption. Further, laser modules in present wavelength division multiplexing (WDM) transmission systems are required to consume several times more power than those in short distance transmission systems since additional power consumption is needed to stabilize the wavelengths of wavelength-tunable light sources.
As for the cost of manufacturing a laser module, it is critical whether an optical isolator is needed or not. Shown in FIG. 12 (Table 1) are the costs of the individual components which constitute an exemplary optical module, namely an optical transceiver module for TTH (Fiber to the Home) applications. As shown in the table, the optical isolator occupies more than a fifth of the total cost. Thus, in addition to the aforementioned miniaturization and power saving efforts, it has become important to make optical modules free of optical isolators by improving the immunity of their laser light sources to reflected light. Typically, an optical isolator is used to prevent the quality of optical signal from degenerating since light emitted from the laser light source may be reflected inside and/or outside the module and return to the lasing cavity. Especially, in the case of a semiconductor laser for long distance transmission applications, an optical isolator is indispensable since the spectral characteristics of the light source are severely required to be stable. From this point of view, FIGS. 2A and 2B summarize present semiconductor lasers, showing their cavity structures and indicating whether they need optical isolators. As understood from the figures, the Fabry-Perot (FP) laser (FIG. 2(a)) and multi transverse mode VCSEL (Vertical Cavity Surface Emitting Laser)(FIG. 2(f)), both for short distance applications, do not need optical isolators. In the case of the distributed feedback (DFB) laser (FIG. 2(b)(c)), distributed Bragg reflector (DBR) laser (FIG. 2(d)) and single transverse mode VCSEL laser (FIG. 2(e), their optical modules are constructed on the condition that optical isolators are to be provided, because the immunity to coherency reflected light is degenerated due to making single mode. The distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers which respectively feature short lasing cavities are described in Japanese Patent Application No. 2005-184588. A well know example of the short lasing cavity FP laser is described in Patent Document 1 (Japanese Patent Laid-Open No. 2004-235182).